This invention relates to glass-ceramics and, more particularly, to glass-ceramics for a substrate of a magnetic information storage medium. The invention particularly provides glass-ceramics which have a high heat resisting property capable of adapting to high temperature film forming and high temperature annealing, super flatness in the surface of a substrate, low solving out of alkali components from the substrate material during high temperature film forming and high temperature annealing, and a low viscosity capable of adapting to press molding on a mass production basis which are particularly suitable for a vertical magnetic recording medium, a patterned medium and a discrete track medium among magnetic information storage medium substrates used in various magnetic information storage devices.
In the present specification, the term “magnetic information storage medium” means a magnetic information storage medium which can be used for a stationary hard disk, a removable hard disk or a card type hard disk used as a hard disk of a personal computer, a hard disk of a digital video camera, a digital camera or an audio device, a hard disk for a car navigation, a hard disk of a mobile phone or a hard disk of various electronic devices.
Recent development of personal computers for multi-media purposes and requirement for digital video cameras and digital cameras to handle a large amount of data such as a moving picture require a magnetic information storage device of a large recording capacity. For increasing the recording density, there is a tendency in the art of a magnetic information storage medium toward reducing the size of a bit cell. As a result, the magnetic head performs its operation in closer proximity to the disk surface.
Moreover, as the recording density has come to exceed 100 Gb/in2, such small magnetization unit becomes thermally instable and hence the surface recording system has reached its limits physically for the requirement for a high recording density exceeding 100 Gb/in2.
For coping with this situation, there is a tendency to adopting the vertical magnetic recording system. Since the easy axis of magnetization runs in vertical direction in this vertical magnetic recording system, the bit size can be reduced significantly. Moreover, since the medium can have a desired film thickness (five folds to tend folds that of the surface recording system) in the vertical magnetic recording system, advantage of reducing a demagnetizing field and advantage of magnetic anisotropy due to configuration can be expected. For these reasons, the vertical magnetic recording system can solve the problems of reduction in recording energy and thermal instability occurring in increasing recording density in the prior art magnetic recording system in the surface direction and realize a recording density which is by far higher than the surface magnetic recording system. Thus, by the vertical magnetic recording system, a recording density of 100 Gb/in2 or over on a practical level has already become possible on a mass production basis and studies are being carried out about a recording density exceeding 300 Gb/in2.
Since magnetization is carried out in vertical direction against the surface of a medium in the vertical magnetic recording system, a medium having the easy axis of magnetization in vertical direction is employed instead of the easy axis of magnetization in the surface direction used in the prior art magnetic recording system. As storage layers used in the vertical magnetic recording system on which studies and efforts for practical implementation are being carried out can be cited a barium ferrite film and various alloy films including Co-γFe2O3, Fe alloys such as FePt and Ni alloys.
It is necessary in such magnetic storage medium to increase a film forming temperature for producing finer crystal grains of a crystal constituting the magnetic substance and causing the crystal to precipitate in vertical direction. Further, according to recent studies, there are cases in which annealing must be conducted at a high temperature in the order of 500° C. to 900° C. for improving magnetic characteristics. Therefore, a substrate material should preferably resist to such high temperature without occurrence of deformation of the substrate and change in the surface roughness.
It is also preferable that such magnetic storage medium should be free of crystal anisotropy, foreign matters and impurities and have a dense, uniform and fine texture. It is also preferable that such magnetic storage medium should have sufficient chemical durability for resisting to rinsing and etching by various chemicals.
In the prior art magnetic disk substrates, aluminum alloy is often used. In the aluminum alloy, however, unevenness in the form of projections or spots tends occur on the surface of the substrate in the polishing process and hence a substrate having sufficient flatness cannot be obtained. Moreover, since the aluminum alloy is soft and tends to be deformed, it is difficult to make the substrate thin. Furthermore, the disk made of aluminum alloy tends to cause a head crush due to deflection during a high speed rotation of the disk resulting in causing damage to a storage medium. Therefore, the aluminum alloy is not a material which can cope sufficiently with the future tendency toward a higher recording density. Besides, since heat resistance of the aluminum alloy during film forming is less than 300° C., film forming at a temperature of 300° C. or over and annealing at a high temperature in the order of 500° C. to 900° C. cause thermal deformation of the substrate. Therefore, it is difficult to use a substrate made of aluminum alloy as a substrate of a magnetic information storage medium requiring treatment at such a high temperature.
As an amorphous glass substrate and a chemically tempered glass substrate are known, for example, substrates of chemically tempered soda lime glass (SiO2—CaO—Na2O) and alumino-silicate glass (SiO2—Al2O3—Na2O). In these substrates, however, heat resisting property of the substrates is low because they are made of amorphous glasses. In other words, there is a problem of deformation due to film forming of a magnetic information storage medium that, when a magnetic information storage medium is provided on these substrates by film forming at a temperature of 300° C. or over, flatness of the substrates is deteriorated. Further, alkali components in the substrates are solved out and cause damage to the film.
Known in the art of glass-ceramics used for a magnetic information storage medium are several glass-ceramics. For example, Japanese Patent Application Laid-open Publication No. 2000-302481 discloses SiO2—Li2O—P2O5 type glass-ceramics comprising lithium disilicate (Li2O.2SiO2) and α-quartz as predominant crystal phases and having excellent physical properties and flatness as a magnetic information storage medium. The glass-ceramics, however, are disadvantageous in that they have a low heat resisting property and cause change in the precipitating crystal phases at a temperature exceeding 500° C. Japanese Patent Application Laid-open Publication No. Hei 09-35234 discloses a substrate for a magnetic information storage medium made of Li2O—Al2O3—SiO2 type glass-ceramics comprising β-spodumene and lithium disilicate. The glass-ceramics likewise have the problem of a low heat resisting property and cause change in the precipitating crystal phases at a temperature exceeding 500° C.
Li2O—Al2O3—SiO2 type glass-ceramics are also known to comprise at least one of β-quartz, β-quartz solid solution, ββ-eucryptite solid solution-eucryptite, β-spodumene and β-spodumene solid solution, have a sufficiently high heat resisting property as a vertical magnetic recording medium. Since, however, the glass-ceramics have a lower value of Li2O/(SiO2+Al2O3), i.e., the ratio of amount of Li2O to the total amount of SiO2+Al2O3 than the glass-ceramics of the present invention, they have a higher viscosity during melting of raw materials and are not suitable for press molding.
It is, therefore, an object of the present invention to provide glass-ceramics having excellent heat resisting property and mechanical property.
It is another object of the invention to provide highly productive glass-ceramics suitable for use as a magnetic disk substrate of a magnetic information storage medium which can be adapted to design and improvement of the above described magnetic information storage device and have good surface characteristic capable of coping with the ramp loading system for recording at a high recording density in both the surface magnetic recording system and the vertical magnetic recording system, have high sufficient strength for coping with high speed rotation and shock by falling, having thermal expansion property which is optimum for respective drive elements and excellent chemical durability, having a low melting temperature and adaptability to press molding.